Techniques for heating and drawing glass into fine fibers have been known for millennia. It was, however, in the 1930s when this technique was used for the textile industry. As explained below, this technique was employed later in the 19th century to make glass optical fibers.
Light guidance in transparent pipes and water streams historically inspired the use of optical fibers for light transmission. The light guiding process using the total internal reflection was first demonstrated by Daniel Colladon and Jacques Babinet in Paris in the early 1840s. It found applications such as illumination in dentistry, image transmission and internal medical examination early in the twentieth century. Later in the 1920s the concept of modern glass fibers with a glass core and a lower index cladding for a more suitable index guiding was introduced. Low-index oils and waxes were mostly used to produce the lower-index cladding. In the 1950s E. Curtiss at the University of Michigan produced the first glass-core fiber with glass cladding in order to minimize the interference of the guided light with the surrounding environment. Advances in the fiber fabrication process and the proper choice of glass materials rendered the optical fibers as feasible tools for long-distance optical telecommunications as well as many other applications such as sensing and imaging. In the 1990s micro-structured fibers and photonic crystal fibers were developed where the guiding mechanism was based on light diffraction from periodic structures in fiber. Photonic crystal fibers could potentially transmit higher light powers and would give the possibility of dispersion adjustment based on structure design. In recent years a new class of fibers (multi-material fibers) emerged based on thermal co-drawing of multiple types of materials all with thermally and mechanically compatible materials. This new class of fibers brought novel functionalities (not limited to optical light transmission) to fibers. An example of this includes fibers with semiconducting glass and metal electrodes integrated into a single fiber for light detection applications. The field of multi-material fibers recently went even further to include piezoelectric fibers and multi-material fibers for structured micro- and nano-sphere fabrication.
Throughout the history of development of fibers, thermal fiber drawing has been the most popular and the most successful fabrication method. Simplicity and speed of thermal fiber drawing made optical telecommunications an economically viable technology. The circularly symmetric geometry of optical fiber fabrication was indeed inspired by the natural shape of water streams and glass fibers that were produced through heating and pulling of glass.
In the fiber drawing process, a softened material has the tendency to round up into fibers with circular cross-section to minimize the surface free energy under surface tension. However, in the longitudinal direction the tension along the fiber, which is produced by the intentional pulling process, dominates the surface tension and leaves the fiber longitudinally elongated. During the pulling process, the material is kept at the softening temperature for a brief period of time, just enough to stretch it into fiber. It is then gradually cooled to solidify the stretched form that is called a fiber. This is the fiber fabrication process that has been used for centuries in the textile industry and decades in optics. In recent years fibers with non-circular cross-sections have been created by giving an asymmetric geometry to the fiber preform and trying to maintain that geometry by not overly heating the fiber during the drawing process. It is possible to maintain non-circular structures by not giving the material enough freedom (low viscosity) and time to round up to a circular shape. Fibers made with this method having hexagonal, square, rectangular and even D-shaped cross-sections have been reported for various applications. For all fibers of different materials for various applications over decades the circular symmetry of fiber preform heating has allowed for equal scale reduction in both transverse directions (height and width) across the fiber. This results in maintaining the aspect ratio of the preform in the final drawn fiber by allowing equal shrinkage in both transverse directions.
The conventional fiber drawing method that has been worked on for about 4 decades involves direct thermal drawing of a scaled up version of the final fiber that is called a fiber preform. More recently, multi-material fibers have been introduced in which a fiber preform has multiple components with various materials all integrated in the scaled up preform prior to the fiber draw as illustrated in FIGS. 1-2. The fiber preform 110 is comprised of a body 112 of a first material with at least one element 114 of a second material embedded therein. FIG. 1 shows two elements 114 of a second material embedded in the body 112. Conventional multi-material fiber drawing relies on matching the thermal and mechanical properties of components of the preforms in a way that, within a common range of temperatures, all components 112, 114 soften to some degree. Therefore, all components 112, 114 will flow together with similar viscosities and will experience a more or less identical or proportional size reduction as illustrated in FIG. 2 where the finished fiber 120 includes the elements 112, 114 with relative sizes substantially proportional to those in the preform 110. In this case, the thermal, mechanical and fluidic properties of all components 112, 114 must match closely in order for the fiber to draw uniformly with all components flow and scale down similarly. This limits the choice of materials and applications of multi-material and multi-component fibers. With conventional fiber drawing methods for any given application or function, it is often not possible to use the best of each class of materials for the specific application while maintaining similar thermal and mechanical properties. In other words, performance of the ultimate device made with such multi-material fibers may be compromised because of the limited choice of materials with matching thermal and mechanical properties.
Therefore, a method of drawing incompatible materials is needed to preclude the necessity of combining materials of like thermal characteristics.